ASTM E385-90(2002)
(Test Method)Standard Test Method for Oxygen Content Using a 14-MeV Neutron Activation and Direct-Counting Technique
Standard Test Method for Oxygen Content Using a 14-MeV Neutron Activation and Direct-Counting Technique
SCOPE
1.1 This test method covers the measurement of oxygen concentration in almost any matrix by using a 14-MeV neutron activation and direct-counting technique. Essentially, the same system may be used to determine oxygen concentrations ranging from over 50 % to about 10 μg/g, or less, depending on the sample size and available 14-MeV neutron fluence rates.
Note 1—The range of analysis may be extended by using higher neutron fluence rates, larger samples, and higher counting efficiency detectors.
1.2 This test method may be used on either solid or liquid samples, provided that they can be made to conform in size, shape, and macroscopic density during irradiation and counting to a standard sample of known oxygen content. Several variants of this method have been described in the technical literature. A monograph is available which provides a comprehensive description of the principles of activation analysis using a neutron generator (1).
1.3 The values stated in either SI or inch-pound units are to be regarded separately as the standard. The values given in parentheses are for information only.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Specific precautions are given in Section 8.
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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:E385–90(Reapproved 2002)
Standard Test Method for
Oxygen Content Using a 14-MeV Neutron Activation and
Direct-Counting Technique
This standard is issued under the fixed designation E385; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope E181 Test Methods for Detector Calibration and Analysis
of Radionuclides
1.1 This test method covers the measurement of oxygen
E496 Test Method for Measuring Neutron Fluence Rate
concentrationinalmostanymatrixbyusinga14-MeVneutron
3 4
and Average Energy from H(d,n) He Neutron Generators
activation and direct-counting technique. Essentially, the same
by Radioactivation Techniques
system may be used to determine oxygen concentrations
2.2 U.S. Government Document:
rangingfromover50%toabout10µg/g,orless,dependingon
Code of Federal Regulations, Title 10, Part 20
the sample size and available 14-MeV neutron fluence rates.
NOTE 1—The range of analysis may be extended by using higher 3. Terminology
neutron fluence rates, larger samples, and higher counting efficiency
3.1 Definitions (see also Terminology E170):
detectors.
3.1.1 accelerator, n—a machine that ionizes a gas and
1.2 This test method may be used on either solid or liquid
electrically accelerates the ions onto a target. The accelerator
samples, provided that they can be made to conform in size,
may be based on the Cockroft-Walton, Van de Graaff, or other
shape,andmacroscopicdensityduringirradiationandcounting
design types (1). Compact sealed-tube, mixed deuterium and
to a standard sample of known oxygen content. Several
tritium gas, Cockcroft-Walton neutron generators are most
variants of this method have been described in the technical
commonly used for 14-MeVneutron activation analysis. How-
literature.Amonograph is available which provides a compre-
ever, “pumped” drift-tube accelerators that use replaceable
hensive description of the principles of activation analysis
tritium-containing targets are also still in use. A review of
using a neutron generator (1).
operational characteristics, descriptions of accessory instru-
1.3 The values stated in either SI or inch-pound units are to
mentation,andapplicationsofacceleratorsusedasfastneutron
be regarded separately as the standard. The values given in
generators is given in Ref (2).
parentheses are for information only.
3.1.2 comparator standard, n—a reference standard of
1.4 This standard does not purport to address all of the
known oxygen content whose specific counting rate (counts
−1 −1
safety concerns, if any, associated with its use. It is the
min [mg of oxygen] ) may be used to quantify the oxygen
responsibility of the user of this standard to establish appro-
content of a sample irradiated and counted under the same
priate safety and health practices and determine the applica-
conditions. Often, a comparator standard is selected to have a
bility of regulatory limitations prior to use. Specific precau-
matrix composition, physical size, density and shape very
tions are given in Section 8.
similar to the corresponding parameters of the sample to be
analyzed. Comparative standards prepared in this way may be
2. Referenced Documents
used directly as “monitors” (see 3.1.4) in order to avoid the
2.1 ASTM Standards:
needformonitor-samplecalibrationplots,inthosecaseswhere
E170 Terminology Relating to Radiation Measurements
the usual monitor reference standard is physically or chemi-
and Dosimetry
cally dissimilar to the samples to be analyzed.
3.1.3 14-MeV neutron fluence rate, n—the areal density of
neutrons passing through a sample, measured in terms of
−2 −1
ThistestmethodisunderthejurisdictionofASTMCommitteeE10onNuclear
neutrons cm s , that is produced by the fusion reaction of
Technology and Applications and is the direct responsibility of Subcommittee
deuterium and tritium ions accelerated to energies of typically
E10.05on Nuclear Radiation Metrology.
150 to 200 keV in a small accelerator. Fluence rate is also
Current edition approved Oct. 26, 1990. Published August 1991. Originally
published as E385–69T. Last previous edition E385–80.
The boldface numbers in parentheses refer to a list of references at the end of
the text. Available from the Superintendent of Documents, U.S. Government Printing
Annual Book of ASTM Standards, Vol 12.02. Office, Washington, DC 20402.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E385
commonly referred to as “flux density.” The total neutron transitionsaredirectlytothegroundstateof O.(Allhalf-lives
fluence is the fluence rate integrated over time. and gamma-ray energies are taken from Ref (5) and decay
3 4
3.1.3.1 Discussion—The H(d,n) He reaction is used to pro- schemes are given in Ref (6).Auseful elemental data base and
duce approximately 14.7-MeV neutrons. This reaction has a calculated sensitivities for 14-MeV instrumental neutron acti-
Q-value of+17.586 MeV. vation analysis (14-MeV INAA) are provided in Ref (7). (See
3.1.4 monitor, n—any type of detector or comparison ref- also Test Methods E181.)
erence material that can be used to produce a response
5. Significance and Use
proportional to the 14-MeV neutron fluence rate in the irradia-
tion position, or to the radionuclide decay events recorded by
5.1 The conventional determination of oxygen content in
the sample detector. A plastic pellet with a known oxygen
liquid or solid samples is a relatively difficult chemical
content is often used as a monitor reference standard in dual
procedure. It is slow and usually of limited sensitivity. The
sample transfer systems. It is never removed from the system
14-MeV neutron activation and direct counting technique
regardless of the characteristics of the sample to be analyzed.
providesarapid,highlysensitive,nondestructiveprocedurefor
In this case monitor-sample calibration plots are required.
oxygen determination in a wide range of matrices. This test
3.1.5 multichannel pulse-height analyzer, n—an instrument
method is independent of the chemical form of the oxygen.
that receives, counts, separates, and stores, as a function of
5.2 This test method can be used for quality and process
their energy, pulses from a scintillation or semi-conductor
control in the metals, coal, and petroleum industries, and for
gamma-ray detector and amplifier. In the 14-MeV INAA
research purposes in a broad spectrum of applications.
determination of oxygen, the multichannel analyzer may also
be used to receive and record both the BF neutron detector
3 6. Interferences
monitorcountsandthesamplegamma-raydetectorcountsasa
6.1 Because of the high energy of the gamma rays emitted
function of stepped time increments (3 and 4). In the latter
in the decay of N, there are very few elements that will
case, operation of the analyzer in the multichannel scaler
produce interfering radiations; nevertheless, caution should be
(MCS) mode, an electronic gating circuit is used to select only
exercised. F, for example, will undergo an (n,a) reaction to
gamma rays within the energy range of interest.
produce N, the same indicator radionuclide produced from
3.1.6 transfer system, n—a system, normally pneumatic,
19 16
oxygen. Because the cross section for the F(n,a) N reaction
used to transport the sample from an injection port (sometimes
16 16
is approximately one-half that of the O(n,p) N reaction, a
connected to an automatic sample changer) to the irradiation
correction must be made if fluorine is present in an amount
station, and then to the counting station where the activity of
comparable to the statistical uncertainty in the oxygen deter-
the sample is measured. The system may include components
mination.Another possible interfering reaction may arise from
to ensure uniform positioning of the sample at the irradiation
the presence of boron. B will undergo an (n,p) reaction to
and counting stations.
produce Be. This isotope decays with a half-life of 13.81 s,
andemitsseveralhigh-energygammarayswithenergiesinthe
4. Summary of Test Method
rangeof4.67to7.98MeV.Inaddition,thereisBremsstrahlung
4.1 The weighed sample to be analyzed is placed in a
radiation produced by the high energy beta particles emitted
container for automatic transfer from a sample-loading port to
by Be. These radiations can interfere with the oxygen deter-
the 14-MeV neutron irradiation position of a particle accelera-
mination if the oxygen content does not exceed 1% of the
tor. After irradiation for a pre-selected time, the sample is
boron present.
automatically returned to the counting area. A gamma-ray
6.2 Another possible elemental interference can arise from
detector measures the high-energy gamma radiation from the
16 thepresenceoffissionablematerialssuchasthorium,uranium,
radioactive decay of the N produced by the (n,p) nuclear
16 and plutonium. Many short-lived fission products emit high-
reaction on O. The number of counts in a pre-selected
energy gamma rays capable of interfering with those from N.
counting interval is recorded by a gated scaler, or by a
40 40 40
multichannel analyzer operating in either the pulse-height, or
NOTE 2—Argon produces an interferent, Cl, by the Ar(n,p) Cl re-
action. Therefore, argon should not be used for the inert atmosphere
gated multiscaler modes. The number of events recorded for
during sample preparation for oxygen analysis. Cl (t ⁄2 =1.35 m) has
samples and monitor reference standard are corrected for
several high-energy gamma rays, including one at 5.88 MeV.
background and normalized to identical irradiation and count-
ing conditions. If the sample and monitor reference standard 6.3 An important aspect of this analysis that must be
sample are not irradiated simultaneously, the neutron dose controlled is the geometry during both irradiation and count-
received during each irradiation must be recorded, typically by ing. The neutron source is usually a disk source. Hence, the
use of a BF neutron proportional counter.The amount of total fluence rate decreases as the inverse square at points distant
oxygen(allchemicalforms)inthesampleisproportionaltothe from the target, and less rapidly close to the target. Because of
correctedsamplecountandisquantifiedbyuseofthecorrected these fluence rate gradients, the irradiation geometry should be
specific activity of the monitor, or comparator standard(s). reproducedasaccuratelyaspossible.Similarly,thepositioning
4.1.1 N decays with a half-life of 7.13 s by b-emission, of the sample at the detector is critical and must be accurately
thus returning to O. About 69% of the decays are accompa- reproducible. For example, if the sample is considered to be a
nied by 6.13-MeV gamma rays, 5% by 7.12-MeV gamma point source located 6 mm from a cylindrical sodium iodide
rays,and1%by2.74-MeVgammarays.Otherlowerintensity (NaI) detector, a 1-mm change in position of the sample along
gamma rays are also observed. About 26% of the beta the detector axis will result in a 3.5 to 5% change in detector
E385
efficiency (8). Since efficiency is defined as the fraction of sealed-tube-type neutron generator obviates the need to handle
gamma rays emitted from the source that interact with the tritium targets and provides for longer stable operation.
detector,itisevidentthatachangeinefficiencywouldresultin
7.2 Sample Transfer System—The short half-life (7.13 s) of
an equal percentage change in measured activity and in the N requires that the sample be transferred rapidly between
apparent oxygen content. Positioning errors are normally
theirradiationpositionandthecountingstationbyapneumatic
minimized by rotating the sample around a single axis, or system to minimize decay of the N. If the oxygen content in
biaxially, during both irradiation and counting. Alternately,
thesampleislow,itisdesirabletousedrynitrogen,ratherthan
dual detectors at 180° can be used to minimize positioning air, in the pneumatic system to avoid an increase in radioac-
errors at the counting station.
tivity due to recoil of N atoms produced in the air onto the
sample surface. The transfer system and data processing may
6.4 Since N emits high-energy gamma rays, determina-
tions are less subject to effects of self-absorption than are be controlled by PC-type microcomputers using programs
written in BASIC (11), or by a minicomputer using programs
determinations based on the use of indicator radionuclides
emittinglowerenergygammarays.Correctionsforgamma-ray written in FORTRAN (4). Dual transfer systems transport the
sample and a monitor reference standard simultaneously. In
attenuation during counting are usually negligible, except in
this case, two independent counting systems are often used.
the highest sensitivity determinations where sample sizes may
Singlesampletransfersystemsbasedonsequentialirradiations
be large.
of a sample and a monitor reference standard, or a comparator
6.5 The oxygen content of the transfer container (“rabbit”)
standard, are also used.
must be kept as low as possible to avoid a large “blank”
correction. Suggested materials that combine light weight and
NOTE 3—As mentioned previously in 6.2, argon should be avoided in
low oxygen content are polypropylene and high-density poly-
the transfer gas, as well as in sample packaging, because of the
ethylene (molded under a nitrogen atmosphere), high purity
interferent Cl produced.
Cu, and high-purity nickel.Asimple subtraction of the counts
7.3 Monitor—The number of counts obtained from any
fromtheblankvialintheabsenceofthesampleisnotadequate
given irradiation is dependent upon the oxygen content of the
for oxygen determinations below 200 µg/g, since large sample
sample, the length of irradiation, the neutron fluence rate, the
sizes may be required for these high-sensitivity measurements
neutron energy spectrum, the delay time between irradiation
andgamma-rayattenuationmaybeimportantwhenthesample
and counting, and the length of the count. It is desirable to
is present (9). If the total oxygen content of the sample is as
make a measurement in which the result obtained is a function
lowasthatofthecontainer(typicallyabout0.5mgofoxygen),
of only the oxygen content and independent of other variables.
the sample should be removed from the irradiation container
This can be achieved by standardizing the experimental con-
prior to counting. Statistical errors increase rapidly as true
ditions and use of a monitor.
sample activities decrease, while container contamination a
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